Heart disease refers to several classes of cardio and cardiovascular disorders and co-morbidities relating to the heart and blood vessels. Generally, heart disease is treatable through medication, lifestyle modification and surgical intervention, which involves repairing damaged organs and tissue. Surgical intervention can also involve implanting active monitoring or therapy delivery devices, such as pacemakers and defibrillators, and passive intervention means.
Heart disease can lead to heart failure, a potentially fatal condition in which the heart is unable to supply blood sufficient to meet the metabolic demands of a body. In a clinical setting, cardiac performance, including potential heart failure, can be detected by measuring changes in arterial blood pressure immediately before, during and immediately after the performance of intrathoracic pressure maneuvers, known as dynamic auscultation, which includes the Valsalva and Müller maneuvers, such as described in Braunwald, “Heart Disease—A Textbook of Cardiovascular Medicine,” pp. 46-52 (5th ed. 1997), the disclosure of which is incorporated by reference.
In particular, potential heart failure can be effectively and safely detected by evaluating the profile of arterial blood pressure and other cardiac dimensional measures relative to performance of the Valsalva maneuver, which involves forced expiration against a closed glottis for about 10-30 seconds. In a healthy person with no prior history of cardiovascular disease or a patient suffering from a diseased but not failed heart, left ventricular ejection fraction (LVEF) and left ventricular end diastolic pressure (LVEDP) change dramatically coincidental to performance of the Valsalva maneuver, whereas LVEF and LVEDP change only slightly in a heart failure patient, as discussed in Hamilton et al., Arterial, Cerebrospinal and Venous Pressure in Man During Cough and Strain, 144 Am. J. of Phys., pp. 42, 42-50 (1944) and Zema et al., Left Ventricular Dysfunction—Bedside Valsalva Maneuver, Br. Heart J., pp. 44:560-569 (1980), the disclosures of which are incorporated by reference.
Circulatory effects and arterial blood pressure profile undergo four well-documented phases during performance of the Valsalva maneuver. During Phase I (initial strain), systemic arterial pressure increases approximately equal to the increase in intrathoracic pressure. During Phase II (strain duration and cessation of breathing), pulse pressure narrows and systemic systolic pressure decreases. During Phase III (strain discontinuation and resumption of normal breathing), systolic pressure drops rapidly. During Phase IV (recovery), diastolic and systolic pressures overshoot and return to pre-maneuver levels. The four phases form a characteristic signature and periodic analysis of arterial blood pressure profile or blood pressure, specifically LVEF and LVEDP, throughout each phase can be indicative of the patient's heart failure status.
Regularly obtaining and evaluating LVEF and LVEDP for chronic cardiac performance assessment, however, can be difficult. Acute direct measurements can be obtained through catheterization and electrodes. LVEF and LVEDP can be measured directly through a catheter distally placed into the left ventricle, but the procedure is invasive and creates unfavorable risks. Pulmonary artery wedge pressure (PAWP), measured through right heart catheterization, can be used as a surrogate measure for LVEDP, but the procedure is also invasive and risky. Moreover, catheterization is impractical in a non-clinical setting. Finally, chronically implanted cardiac pressure electrodes, while less risky, are generally inaccurate and unreliable. Consequently, indirect measurements approximating LVEF and LVEDP are preferable for chronic cardiac assessment.
For example, intracardiac impedance is readily measured through cardiac impedance plethysmography and can be used as an indirect measure of LVEF and LVEDP. Changes in intracardiac impedance correlate to cardiac dimensional changes, such as described in McKay et al., Instantaneous Measurement of Left and Right Ventricular Stroke Volume and Pressure-Volume Relationships with an Impedance Catheter, Circ. 69, No, 4, pp. 703-710 (1984), the disclosure of which is incorporated by reference. Similarly, cardiac vibrations, or heart sounds, can be measured through the use of an acoustic microphone or accelerometer. The energy of the first heart sound is proportionate to the rate of rise of left ventricular pressure, which is proportionate to cardiac preload. As a result, by measuring intracardiac impedance or cardiac vibrations, a profile of LVEDP and LVEF response during performance of the Valsalva maneuver can be obtained indirectly without resorting to invasive direct measurement techniques. Similarly, changes in cardiac filling can be determined from the third and fourth heart sounds. Known plethysmography techniques for indirectly measuring intracardiac impedance, however, adapt poorly to effective chronic long-term monitoring.
U.S. Pat. No. 4,548,211 to Marks discloses the use of external admittance impedance plethysmography to measure pulsatile volume and net inflow in a limb or body segment. External electrodes are placed on the skin and a voltage is applied and sensed for use in determining absolute physiologic values of peak-to-peak pulsatile volume and peat net inflow. While instrumental in non-invasively measuring peripheral blood flow dynamics, the Marks device fails to measure or monitor cardiac dimensional changes through intracardiac impedance.
U.S. Pat. No. 5,788,643 to Feldman discloses a process for monitoring patients with chronic congestive heart failure (CHF) by applying a high frequency current between electrodes placed on the limbs of a patient. Current, voltage and phase angle are measured to calculate resistance, reactance, impedance, total body water and extracellular water, which are compared to a baseline for identifying conditions relating to CHF. The Feldman process is limited to operating on external peripheral electrodes and fails to measure or monitor cardiac dimensional changes through intracardiac impedance.
U.S. Pat. Nos. 6,120,442 and 6,238,349 both to Hickey disclose an apparatus and method for non-invasively determining cardiac performance parameters, including systolic time intervals, contractility indices, pulse amplitude ratios while performing the Valsalva maneuver, cardiac output indices, and pulse wave velocity indices. A catheter is inserted into the esophagus and a balloon is pressurized at a distal end, positioned adjacent to the aortic arch to sense aortic pressure. The affects of aortic pressure on the balloon are utilized to determine the cardiac performance parameters. The Hickey devices, while capable of assessing cardiac performance, must be performed in a clinical setting and fails to measure or monitor cardiac dimensional changes through intracardiac impedance.
U.S. Pat. Nos. 3,776,221 and 5,291,895 both to McIntyre disclose a pressure-sensing device for providing a signal representative of systemic arterial blood pressure before and after performance of the Valsalva maneuver. Specifically, an electrode placed on the skin generates a blood pressure signal and measures changes in amplitude before and after the Valsalva maneuver is performed. The McIntyre devices are limited to operating with external skin electrodes and fail to measure or monitor cardiac dimensional changes through intracardiac impedance.
Therefore, there is a need for an approach to assessing cardiac performance by indirectly measuring arterial blood pressure profile through intracardiac impedance or heart sounds, that is, cardiac vibrations, recorded relative to performance of intrathoracic pressure maneuvers, such as the Valsalva maneuver. Preferably, such an approach would analyze an intrathoracic pressure maneuver signature in a non-clinical setting on a regular basis for use in automated heart disease patient management.